1. Field of the Invention
This invention relates to the preparation of steel strip and sheet stock for hot dip coating with molten coating metal and more particularly to a method of preparation involving initial development of an oxide film on the stock surfaces, preservation of the oxide film during further heating, and reducing the oxide film while cooling the stock prior to immersion thereof in a molten coating metal bath. The invention has particular utility in the coating of carbon steels, low carbon rimmed and aluminum killed steels and low alloy steels, with aluminum, zinc, alloys of aluminum, alloys of zinc, and terne.
2. Description of the Prior Art
In the fluxless hot dip metallic coating of steel strip and sheet stock it is necessary to subject the surfaces to a preliminary treatment which provides a clean surface free of iron oxide scale and other surface contaminants, and which is readily wettable by the molten coating metal in order to obtain good adherence. Two types of in-line anneal preliminary treatments are in common use in this country, one being the so-called Sendzimir process or oxidation-reduction practice (disclosed in U.S. Pat. Nos. 2,110,893 and 2,197,622, to T. Sendzimir), and the other being the so-called Selas process or high intensity direct fired furnace line (disclosed in U.S. Pat. No. 3,320,085 to C. A. Turner, Jr.).
In the Sendzimir process steel strip or sheet stock is heated in an oxidizing furnace to a temperature of about 370.degree.-485.degree. C. without atmosphere control, withdrawn into air to form a controlled surface oxide layer varying in appearance from light yellow to purple or blue-grey, introduced into a reduction furnace containing a hydrogen and nitrogen atmosphere wherein the stock is heated to about 735.degree.-925.degree. C. and the controlled oxide layer is completely reduced. The stock is then passed into a cooling section containing a hydrogen and nitrogen atmosphere, brought approximately to the temperature of the molten coating metal bath, and then led beneath the bath surface while still surrounded by the protective atmosphere.
In the Selas process steel strip or sheet stock is passed through a direct fired preheat furnace section, heated to a temperature above 1315.degree. C. by direct combustion of fuel and air therein to produce gaseous products of combustion containing at least about 3% combustibles in the form of carbon monoxide and hydrogen, the stock reaching a temperature of about 425.degree.-705.degree. C. while maintaining bright steel surfaces completely free from oxidation. The stock is then passed into a reducing section which is in sealed relation to the preheat section and which contains a hydrogen and nitrogen atmosphere, wherein it may be further heated by radiant tubes to about 425.degree.-925.degree. C. and/or cooled approximately to the molten coating metal bath temperature. The stock is then led beneath the bath surface while surrounded by the protective atmosphere.
British patent specification No. 1,170,057 published Nov. 12, 1969, discloses a method of preliminary treatment wherein steel strip or sheet is passed through a direct-fired first furnace in which the fuel-to-air ratio is controlled to provide an atmosphere which is slightly oxidizing to the steel, thereby producing an iron oxide layer of a thickness not greater than 10.sup.-5 inches, and is thereafter passed through a second furnace having an atmosphere which is mildly reducing, i.e., sufficient to reduce the iron oxide layer. In the first furnace the oxygen content is not more than 10.sup.-9 atmospheres oxygen pressure (about 10.sup.-7 percent). The atmosphere in the second furnace is maintained at not more than 15% hydrogen and balance inert gas. The first furnace is heated to a temperature of 1100.degree. to 1500.degree. C., and the temperature of the strip or sheet exiting therefrom is 400.degree. to 950.degree. C.
U.S. Pat. No. 3,936,543, issued Feb. 3, 1976, to F. Byrd et al., discloses an improvement in the Selas process, resulting in higher combustion efficiency and better production rates, wherein strip and sheet stock is heated to about 540.degree. to 705.degree. C. in a direct fired preheat furnace section heated to at least about 1205.degree. C. and containing gaseous products of combustion ranging from about 3% by volume oxygen to about 2% by volume excess combustibles in the form of carbon monoxide and hydrogen, followed by heating in a reducing section containing at least about 5% hydrogen by volume (preferably at least 15%) to a temperature of at least about 675.degree. C. Preferably the preheat furnace atmosphere contains 0% oxygen and 0% excess combustibles, i.e., perfect combustion. In this process the stock is heated above the critical strip temperature (i.e., that at which iron oxide is formed), and an iron oxide layer is formed having a thickness of the same order of magnitude (10.sup.-5 inches) as in the above mentioned British patent.